The replacement of defective heart valves with hemodynamic prostheses is the most prevalent course of treatment for certain types of heart disease and dysfunction affecting the atrioventricular valves--namely the right AV (tricuspid) and the left AV (bicuspid) valves. Although a variety of tissue and prosthetic heart valve mechanisms have been developed, monoleaflet (tilting disc) and bileaflet valves currently hold the greatest measure of acceptance among practitioners. These valves include one or two pivoting leaflets or occluders retained within a seating collar or suture ring that is implanted in place of the physiological valve.
Replacement of a bicuspid (mitral) valve using a procedure that preserves portions of the papillary muscle and chordal apparatus is discussed herein for exemplary purposes. In that procedure, the anterior leaflet is bisected and detached from the annulus, and the two halves are groomed and then sutured to the posterior mitral annulus with the papillary muscle and chordal apparatus substantially intact. Such a procedure and its benefits are described in significant detail by H. Feikes, et al., Preservation of All Chordae Tendineae and Papillary Muscle During Mitral Valve Replacement with a Titling Disc Valve, 5 J. Cardiac Surg., No. 2 pp. 81-85 (1990). The authors conclude that this mitral valve replacement procedure can be practical using both monoleaflet and bileaflet valves. However, it is readily apparent to those skilled in reconstructive cardiac surgery that selection of a suitable valve type and proper orientation of the prosthesis can be important factors impacting the long term success of this procedure for a given patient. In particular, due to the position at which the valve tissue is sutured to the posterior mitral annulus, care must be taken to ensure that the peripheral edge of a leaflet does not contact the tissue during normal operation of the valve. Such contact can result in the intermittent, partial, or complete malfunction of the valve, as well as damage to or dislodgement of the valve tissue.
Four primary combinations of valve types and orientation are considered, as diagramed in FIGS. 25-28 herein. The four combinations ranked by ascending level of risk include: (1) monoleaflet valve M with anterior orientation (FIG. 25); (2) bileaflet valve with anti-anatomical orientation (FIG. 26); (3) bileaflet valve with anatomical orientation (FIG. 27); and (4) monoleaflet valve M with posterior orientation (FIG. 28). While the monoleaflet with posterior orientation is generally regarded as a high risk configuration and the monoleaflet with anterior orientation is considered to have little or no risk, the degree of risk associated with a bileaflet valve oriented in either the anatomical or anti-anatomical configuration depends upon the particular type of valve selected (particularly its range of excursions, radial exposure, and lateral exposure), the post-procedure anatomical characteristics of the annuls, and the patient's requirement for certain operational parameters associate with the valve.
While a monoleaflet valve may be preferred in order to achieve the lowest risk level with an anterior orientation, a physician may prefer to implant a bileaflet valve to obtain specific functional benefits associated with or unique to the particular bileaflet valve structure.
The bileaflet valve has been extensively developed and refined. However, there is still room for further improvement. Problems associated with the weakening or structural failure of critical components in the valve are linked both to dynamic mechanical stresses and cavitation. It is noted that a certain amount of antegrade and retrograde leakage is generally anticipated. However, the amount of leakage is preferably maintained within acceptable limits corresponding roughly to normal anatomical valves. In addition, minimizing the physical size of the valve prosthesis, particularly the longitudinal dimensions of the annular base, produces greater excursion along the peripheral edges of the leaflets, while simultaneously increasing the difficulty in raising the heights of the pivot axis. Furthermore, recesses, crevices, corners, and obstructions required to restrain the leaflets within the annular base and maintain pivotal movement also interfere with circulation, create turbulence, and produce zones of stagnation, each potentially providing a thrombogenic nidus that may eventually lead to an embolism. Although bileaflet valves are hemodynamic, spacing the fixed axis of rotation of the leaflets significantly apart from the secondary natural axis of rotation limits the maximum speed or angular rate which the leaflets may attain during opening and closing.
In regard to the selection of suitable materials, there is an inherent balancing between the selection of materials for ease of fabrication, biocompatibility, strength, and weight versus selection with respect to the acceptable level of fragility of the resulting components, particularly those involving delicate structures such as wire guides, cages, and pins that bear significant loads. In addition, the structure of many pivot mechanisms requires the annular bases to have opposing flat sides rather than a substantially or completely circular bore, thereby restricting the maximum flow volume and increasing the valve's nominal fluid pressure.
U.S. Pat. No. 4,276,658 to Hanson provides a representative example of a conventional bileaflet heart valve. That valve utilizes a pair of semicircular pivot "ears" disposes on opposing sides of each leaflet received within "hourglass-shaped" slots to control the pivotal movement of the leaflets--including the angular sweep between the open and closed positions, the tiling of the valve away from its restrained pivotal axis, and the translational movement of the leaflet both parallel with its normal plane and along the linear flow path through the bore of the annular base. The Hanson '658 patent also describes the use of a pyrolytic carbon coating over a metallic or synthetic substrate for fabrication of the valve's components.
For comparison, U.S. Pat. Nos. 4,240,161 to Huffstutler and 3,859,668 to Anderson provide representative examples of the features, structure, and operation of monoleaflet or "titling disc" heart valves.
Various improvements directed toward correcting the deficiencies described above have been developed, each achieving varying degrees of success and accompanied by inherent tradeoffs with other beneficial features.
U.S. Pat. No. 3,903,548 to Nakib discloses an effort to utilize the beneficial features of the monoleaflet principle in a bileaflet valve that similarly omits fixed pivotal axis, however the resulting cage structure produces an unacceptably small effective bore and correspondingly high pressure gradient across the valve.
In a bileaflet valve structure such as disclosed in the Hanson '658 patent, the leaflets may each pivot filly between the open and closed portions on the order of 80,000-120,000 times per day given a standard pulse of 60-80 beats per minute. Movement of the leaflets through a viscous aerated fluid such as blood may produce significant cavitation --the formation of partial vacuums caused by sudden movement of the flowing fluid away from the surface of the leaflets as a result of mechanical forces exerted by the leaflets. These partial vacuums produce "micro bubbles" on or near the surface of the leaflets, and when the pressure is released, vacuums change into positive pressure regions which lead to implosion of bubbles which can cause pitting of the surface of the leaflet. The cavitation potential is amplified greatly by the virtually instantaneous stopping and starting of the leaflets as they contact a rim along the annular base and also, in the case stopping, by the rate of speed at which the leaflet is traveling when it stops. Contact between the leaflet and the rim greatly increases the compressive forces on the adjacent fluid, and as the leaflet pivots away from the rim the corresponding effects of the expansions are magnified by increased negative pressures and stronger partial vacuums. Whereas standard cavitation produces pitting of metal surfaces due only to mechanical contact between the flowing fluid and moving object, introducing reciprocal movement and mechanical contact within the fluid cause the collapsing cavitation bubbles to strip or shear material from the leaflet surfaces at an accelerated rate. Although the surface pitting occurs at a near microscopic level, the result is surface degradation of the leaflet which can induce stress fractures and fragmentation leading to the premature failure of a leaflet.
U.S. Pat. No. 4,078,268 to Possis discloses a substantially circular bore through the annular base, as well as a nearly complete separation between the peripheral edges of the leaflets and the annular base around the circumference of the valve. While this design obviates certain cavitation problems, it permits high levels of antegrade and retrograde leakage and places the entire load of restraining each leaflet on a pair of pivot pins received within adjustable bearing plugs. The combination of increased torque, absorbed impact forces, vibration, and normal frictional contact are believed to exert undue mechanical stresses on the relatively delicate pivot pins and bearing plugs.
U.S. Pat. No. 5,080,669 to Tascon discloses an annular base that defines channels which intersect the pivot axis of the leaflets at various angles to direct flow of blood around enlargements in the leaflets that serve as the pivot axis, in an effort to cleanse the surfaces of the enlargements and prevent zones of thrombogenic stagnation from forming. However, the inward projections forming the channels and barriers restraining the leaflets in the Tascon '669 design create obstacles to uniform blood flow through the bore of the annular base, and define acute corners and crevices which can accelerate the formation of a thrombus. In addition, the enlargements continuously block a majority of the potential flow through each of the channels, thereby minimizing any cleansing effect that is realized.
U.S. Pat. No. 4,892,540 to Vallana discloses a pair of vertical "chimneys" defined by the lobes of the annular base and communicating with the recesses in which the ears of the respective leaflets are received. In concept, blood flow in either the antegrade or retrograde direction passes between the pivot ears and the side wall of the annular base to cleanse the recess. However, the angled base portions forming each wedge-shaped separator body hold the pivot ears and leaflets in an elevated position proximate to the inlet from the chimney into the recess, thereby minimizing flow through the chimney. The pivot ears either reduce the flow rate within the recess or divert the flow away from portions of the recess where stagnation could occur, thus diminishing the effectiveness of any cleansing action. Whereas Tascon '669 contemplates alternating between multiple flow paths oriented at diverse angles to enhance the "scrubbing" effect, Vallana '540 only contemplates cleansing that is substantially repetitive and reciprocal along one path for both antegrade and retrograde flow. Finally, to the extent that Vallana '540 would produce an acceptable retrograde cleansing action due to the pressure differential created within the recess feeding into the chimney, it is at the expense of a significantly restricted non-circular bore through the annular base accounting for a substantial reduction in antegrade circulation.
Although the Hanson '658 patent discloses the pivot ears preventing blood stagnation in the area of engagement with the recesses, the use of transesophageal echocardiography in patients receiving mitral valve replacements has shown the formation of dangling fibrin strands along the interior surface of the valve in the areas between and proximate to the pivot recesses. These small filamentous abnormal echoes (SAE) are considered non-obstructive while within the valve, however their frequent disappearance strongly suggests a thrombotic origin and a significant correlation with the risk of early thrombogenic episode has been observed.
Many factors may be responsible for the formation of the fibrin strands, including regions of blood stagnation which provide a nidus for thrombogenic formations, or defects in the materials or structure of the valve that permit the direct attachment of blood cells. It may therefore readily be appreciated that two important goals when designing a bileaflet heart valve are maintaining optimal antegrade and retrograde circulation, and eliminating regions of reduced circulation within the valve that might foster the development of a thrombogenic mass. It is suggested that while the Hanson '658 patent shows a relatively shallow semi-circular recess, in practice it has not been possible to achieve a workable commercial embodiment of a bileaflet valve having pivot ears with a suitably shallow recess to enhance cleansing of the recess by normal antegrade and retrograde circulation. For example, the commercially available embodiments of the Hanson '658 valve have recesses forming entrance angles ranging from 35.degree. to 48.degree. measured between the lateral wall of the bore and the tangentially adjoining surface of the recess, depending upon overall size of the valve. Recesses forming an angle of 35.degree. or less with the adjoining lateral wall have been achieved in monoleaflet valves, however the significantly different structure and operation of monoleaflet valves has not permitted the successful utilization of many comparable features in bileaflet valves.
Various adaptations have also been made in an effort to improve the pivot mechanism. One option is to eliminate the pivot ears or pins, and allow the leaflet to rock on projections extending inwardly from the annular base. These configurations generally require some engagement between the leaflet and the projections--either the projection being received within a notch or recess in the leaflet, or the leaflet forming a trapping flange that prevents egress from between two spaced-apart projections. For example, U.S. Pat. Nos. 4,863,459 to Olin and 4,935,030 to Alonso describe leaflets that include a swelled area or camming surface trapped between two projections. U.S. Pat. Nos. 4,373,216 to Klawitter, 4,692,165 to Bokros, 4,872,875 to Hwang, and 5,354,330 to Hanson each describe a variation in which the leaflet defines a peripheral notch or recess receiving a projection the annular base. While designs utilizing a notch in the leaflet are more secure than the trapped flange configurations, they are also more difficult to assemble without placing undue stress on the leaflets or projections. In addition, these designs similarly present flat-sided bores and projections which extend into the bore and obstruct antegrade flow. As the complexity of these projections increases, the opportunity for a crevice or recess providing a thrombogenic nidus also increases. Representative examples of relatively complex pivot structures that present several potential stagnation sites include U.S. Pat. Nos. 5,116,367 to Hwang and 5,123,920 to Bokros.
One prominent feature of the bileaflet valves discussed above is the degree of exposure or incursion that is exhibited by the leaflets relative to the annular base. Excursion can be thought of as the maximum distance which the distal ends of the leaflets protrude from the bottom of the annular base when the valve is completely open, measured from the lowermost planar surface of the base to the most distal point on the peripheral edge of the respective leaflet. However, when comparing the anatomical and anti-anatomical orientation of a bileaflet valve with reference to the mitral valve replacement procure discussed above, incursion can also encompass two more complex relationships.
U.S. Pat. Nos. 5,246,453 to Bokros and 5,002,567 to Bona disclose alternate configurations in which the leaflets are not generally planar, and are supported by and pivot about fulcrums disposed on the lower portion of each leaflet. While these designs present an incursion both above and below the annular base, it allows the height of the annular base to be reduced somewhat relative to comparable bileaflet valves. While such a design is considered to be more responsive to reversal in the antegrade flow, it also relies upon shifting the axis of rotation relative to the leaflet's moment of inertia and therefore produces different operational characteristics than might normally be expected.
One factor previously alluded to which affects the speed at which the valve operates, is the displacement between the fixed axis of rotation and the corresponding En moment of inertia of the leaflet. Another factor is the shape of the leaflet. In this regard, optimization of several physical parameters must be contemplated. The leaflets must move through an arcuate path in response to fluid pressure applied from both the antegrade and retrograde directions, starting from differential initial orientations relative to the fluid pressure, and within an initially static versus initially dynamic environment. Consequently, valves having superior opening characteristics may be slow to close or resist complete closure, and vice versa. Leaflets having an angled, curved, or bicurved design to enhance the immediate responsiveness to changes in hemodynamic forces can be employed to address this problem. Other factors include reducing turbulence or backwash that might resist the leaflet's momentum or increase its apparent resting inertia, reducing the weight or thickness of the leaflet, allowing the leaflet to rock or cam differently in response to antegrade or retrograde pressures, maximizing the laminar flow through the valve body over the entire leaflet surface, and eliminating sources of friction, vibration, or misalignment that could adversely affect the mechanical operation of the valve.
Another approach mentioned above is to increase the translational movement of the leaflet within the annular body, thereby permitting the leaflet to pivot more naturally about its inertial axis in direct response to the hemodynamic forces. This approach can potentially be more beneficial than merely moving the fixed axis of rotation nearer to the moment of inertia, since it also serves to reduce frictional forces and other physical impediments to proper valve operation. One limitation is the need to maintain proper alignment and seating of the leaflet without encumbering the flow passage with obstructions or incorporating free structures that increase the likelihood of valve failure.
U.S. Pat. No. 4,535,484 to Marconi describes a bileaflet valve in which the leaflets are "free-floating", thereby increasing translational movement and reducing the mechanical stresses imposed at localized pivot points and other load bearing surfaces. However, the Marconi '484 design requires a complex and fragile cage structure to restrain the leaflets, thereby producing a significant risk of damage to the valve during manufacturing or handling and increasing the potential for catastrophic failure of a valve component that would result in death or severe injury to the patient, mitigating against the use of certain materials such as pyrolytic carbon, and greatly increasing the cost and complexity of fabrication.
For comparison, U.S. Pat. No. 4,689,046 to Bokros describes a trapezoidal pivot ear having beveled edges, arguably decreasing the translational freedom, but enhancing the "sweeping" effect of the pivot ear to prevent thrombogenic formations within the recesses and distributing lateral stresses over a wider surface area.
It will also be appreciated from analyzing bileaflet heart valves, such as disclosed by the Hanson '658 and Possis '268 patents, that the leaflets divide the bore into three passages having unequal cross-sectional areas, and that corresponding effects on fluid dynamics should be expected. Observation of these valves in operation shows that flow rates through the passages will vary generally inversely with the corresponding cross-sectional area. As such, in a valve such as Hanson '658 which present a relatively narrow central passage, the flow rate of blood passing through that central passage is greater than through the two passages on opposing sides. The faster blood flow in the center, relative to the sides, can cause additional turbulence within or downstream of the valve, or produce a pressure differential or venturi effect within the valve that can impede or retard the optimal translational or pivotal movement of the leaflets. The Possis '268 valve presents a larger central passage with narrower cross-sectional passages on each side, thereby reversing the fluid dynamics compared with the Hanson '658 design.
While many common functional goals have been recognized among designers of bileaflet heart valve prostheses, there are strongly divergent opinions concerning the prioritization of those goals and how best to achieve specific results or advantages. Accordingly it will be readily appreciated that these competing factors significantly influence the design and optimization of all bileaflet heart valves and that further improvements may be made. The present invention provides advantages over the prior art bileaflet heart valves and solves problems associated therewith.